The use of electric or electromagnetic fields to promote healing, particularly the healing of fractured bones, has been investigated since the early 19th century. See Spadaro, "Bioelectric Stimulation of Bone Formation: Methods, Models and Mechanisms," J.Bioelectricity, 1, 1, 99-128 (1982). In early research, direct current techniques were used by applying electrodes to the skin or via the use of implanted electrodes into the bone. More recently, research has focused on mechanisms which encourage growth through the use of electromagnetic fields to induce voltage and current effects within the tissue. These techniques have been particularly useful in treating non-healing or "non-union" fractures by inducing healing of bones which do not readily heal naturally.
An example of a technique for the use of electromagnetic radiation to promote bone growth is illustrated in U.S. Pat. No. 4,266,532 to Ryaby et al. This patent shows an effective technique for promoting bone growth. However, the technique disclosed by Ryaby et al. requires the use of power supplied from a standard wall socket. Electrotherapy is only useful as long as the patient uses it. Being tethered to the wall is a sufficient annoyance such that many patients will not follow the electrotherapy regimen prescribed by their doctors. Therefore, it is desirable to provide an electromagnetic therapy system which is completely portable.
The apparatus illustrated in U. S. Pat. No. 4,574,809 to Talish et al. is a portable electrotherapy system. The Talish et al. system includes a mounting structure which allows the user to recharge the batteries in the therapy device by connecting the device to a base section which contains recharging circuitry. However, because of the small size of the housing for batteries in such a device, and because of the high energy requirements of electrotherapy, the batteries of the Talish et al. device must be recharged often. This also is often of sufficient inconvenience to many patients to cause noncompliance with the required therapy regimen.
It is impractical to try to provide for extended battery life by enlarging the size of the battery, since that merely enlarges the size of the electrotherapy device which the patient wears and increases its weight and thus effectively defeats the portability feature, since many patients will simply not use a device which is bulky, heavy or inconvenient to wear or carry.
The most effective way of making a battery-powered electrotherapy device sufficiently small and light that it is easily portable and yet be therapeutically effective in practice is to enhance the service life of the battery. This permits the patient to get prolonged treatment without having to change or recharge the device's battery. Maintaining the small size in combination with the enhanced battery life makes the device much more acceptable to patients, and consequently they will be more willing to wear or use the device for the prescribed treatment period.
One method of enhancing battery lifetime while yet retaining the desirable small battery size is to recover the energy stored in the electromagnetic transducer of the electrotherapy system. Generally, the transducers used in electrotherapy to produce the electromagnetic radiation are highly inductive. Because a great deal of the energy applied to these inductive transducers is stored in the transducers in the form of magnetic energy, it would be desirable to recover that energy and direct it back to the control circuitry of the electrotherapy system for reuse. One possible system to recover such energy is shown in U.S. Pat. No. 4,654,574 to Thaler. The elaborate method shown in Thaler involves the use of ancillary energy recovery coils which are inductively coupled to the transducer coil. During the collapse of the magnetic field induced by the inductor, an electromotive force (EMF) is generated in the energy recovery coil which is then applied to a storage capacitor. However, energy recovery in the Thaler system is limited by the coupling between the transducer coil and the energy recovery coil and by losses in the energy recovery coils.
It would therefore be of substantial value to have an electrotherapy device which is sufficiently small and light to be portable, and in which battery life is sufficiently long that users do not find frequency of battery replacement to be of concern.